1. Field of the Invention
The present invention relates generally to computer-aided design of vehicles and, more specifically, to a system and method for replacing parametrically described surface features with independent surface patches in the computer-aided design of a vehicle.
2. Description of the Related Art
Vehicle design, and in particular the design of an automotive vehicle, has advanced to a state in which computer-aided design techniques are frequently incorporated in the development of a new vehicle or redesign of an existing vehicle. Enhanced visualization software tools allow for interactive display and manipulation of large geometric models, including models developed using computer-aided design (CAD). In the field of vehicle design, the use of computer-aided design and visualization techniques are especially beneficial in designing, packaging, and assembling various systems incorporated within the vehicle to maximize the design and functional capabilities of these vehicles.
Certain parametrically described or mathematical surface features, referred to as Direct Surface Manipulation (DSM) features, are capable of describing complex features on existing mathematical surfaces. An example of DSM is disclosed in U.S. Pat. No. 5,903,458 to Stewart et al. for a system and method for forming geometric features using global reparametrization, which disclosure is hereby incorporated by reference. In this patent, an improved DSM method is disclosed which incorporates a global surface reparametrization scheme for the purposes of extending the DSM method to applications involving multiple surfaces and for reducing the geometric effect of parametric spaced distortions on the features. The method reparametrizes multiple surface patches with a shared two-dimensional space defined in the object space of the model. The result is a geometrically consistent mesh, called a super-mesh that serves as a global, uniform parametric space for topologically-disconnected, geometrically-disproportional surface patches. Spherical projection is employed to perform the patch reparameterization. This allows the creation of DSM features on multiple surface patches with improved shape quality.
A limitation of the DSM features, however, is that the underlying mathematics is different than that of the original surface formed from the industry's standard surface representation, i.e., the Non-Uniform Rational B-spline (NURBS). The resulting model is not recognized by standard CAD systems.
Although many commercial CAD systems routinely use non-standard representations as internal, intermediate construction tools for shape creation, the result is typically converted into a NURBS representation for internal consistency and external data exchange. Due to the complexity of the mathematics involved, however, it is generally impossible to analytically translate a DSM representation into a NURBS representation. Hence, the conversion must be done numerically.
The most commonly used numerical method applicable to solving this problem is surface fitting. Surface fitting has been classically concerned with fitting a surface through a cloud of data or sampled points. The data is usually collected by scanning a physical prototype—a process that often includes noise with the measured data. The data points are then fitted using a mathematical surface patch, such as a NURBS surface. The control points of the fitted surface patch are typically located with a least-squares approach, which reduces data and helps to filter out the noise. A least-squares approximation also reduces the oscillations that occur with some polynomial surfaces. However, since the accuracy of fit is measured at the discrete data points, the oscillations are not eliminated.
Therefore, it is desirable to convert DSM features into a standard portion of a recognizable surface patch. It is also desirable to provide a system and method to allow certain types of geometric surface features to be replaced accurately with separate surface patches. Thus, there is a need in the art for a system and method for fitting a DSM feature with a NURBS surface patch and replacing DSM features with independent NURBS surface patches for a CAD model.